Flowmeter for two-phase fluid with simultaneous or alternating measurement of the gas phase and the liquid phase

ABSTRACT

The invention relates to a flowmeter for liquid/gas two-phase cryogenic fluids, including: a separator of the liquid/gas phases, preferably made up of vessel, the cryogenic fluid being admitted into the top portion of said vessel; a liquid flow sensor, located on a liquid pipe in fluid communication with the bottom portion of the vessel, the vessel being placed in the top position in the space relative to the liquid flow sensor; a gas pipe, in fluid communication with the upper portion of the vessel, provided with a flow sensor of the gas phase circulating in said gas pipe; a three-way valve capable of recovering, in two of the channels thereof (A/B), the downstream end of said gas pipe and the downstream end of said liquid pipe; and a device for measuring the level of liquid in the vessel, preferably comprising two level sensors: a bottom level sensor and a top level sensor.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. applicationSer. No. 15/038,408, filed May 20, 2016, which is a § 371 ofInternational PCT Application PCT/FR2014/052893, filed Nov. 13, 2014,which claims § 119(a) foreign priority to French patent applicationFR1361472, filed Nov. 21, 2013, all of which is being incorporated byreference herein in its entirety for all purposes.

BACKGROUND Field of the Invention

The present invention relates the field of flowmeters for gas/liquidtwo-phase fluids.

Related Art

Measuring the flow rate of a two-phase fluid composed of a liquid and agas is a difficult operation when it is desired to measure a mass flowrate. Indeed, all sensors that measure a flow rate are disturbed whenthey are brought into contact with a two-phase liquid, the density ofwhich changes at any moment. This is in particular valid for measuringthe flow rate of cryogenic fluids such as liquid nitrogen.

Certain flowmeters listed in the literature are based on the measurementof the velocity of the fluid. These are for example:

-   -   turbine flowmeters: a turbine is installed in the fluid in        motion and the rotational speed of the turbine gives a        representation of the velocity of the fluid.    -   Pitot tube flowmeters: two tubes are installed in the fluid in        motion to be measured. One tube is installed perpendicular to        the flow and gives the static pressure, the other is installed        parallel to the flow and gives the total dynamic pressure. The        dynamic pressure difference between these two measurements makes        it possible to calculate the flow rate.    -   Ultrasonic flowmeters: some use the Doppler effect (analysis of        the frequency reflected by the particles of the fluid that gives        a representation of the velocity of the particle and therefore        of the fluid) while others measure a difference in transit time        of an ultrasonic wave from upstream to downstream and from        downstream to upstream (representation of the velocity of the        fluid).

In all these cases, when the density of the fluid varies continuously,the change from volume flow rate to mass flow rate is difficult to carryout accurately.

Other systems use the measurement of head loss (pressure loss) in orderto deduce the flow rate therefrom. These are for example calibratedorifice flowmeters that measure the head loss upstream and downstream ofa calibrated orifice placed in the fluid in motion. The measurement fromthese devices is highly disturbed when the fluid does not have aconstant density and when the content of gas increases in the liquid.

Electromagnetic flowmeters, applicable only to fluids having asufficient electrical conductivity, use the principle of electromagneticinduction: an electromagnetic field is applied to the fluid and theelectromotive force created (force proportional to the flow rate of thefluid) is measured. In the case of measuring the flow rate of(nonconductive) cryogenic fluids such as liquid nitrogen, this principleis not applicable.

Vortex flowmeters are based on the phenomenon of generation of vorticesthat are observed behind a fixed bluff body placed in a fluid in motion(Karman effect). Measuring the pressure variations created by thesevortices gives the frequency of the vortices, which is itselfproportional to the velocity of the fluid when the fluid retainsconstant properties. When the density of the fluid varies, themeasurement is distorted.

Thermal flowmeters are those based on measuring the increase intemperature created by a constant supply of energy. A system with twotemperature probes measures the temperature difference between the flowentering and leaving the flowmeter. Between these two probes, aresistance heater provides a known amount of energy. When the heatcapacity of the fluid in motion is known, the flow rate may becalculated from these measurements. However, this principle is notapplicable to two-phase liquids, of which the thermal behavior(vaporization of the liquid) is completely different from single-phaseliquids.

Only a Coriolis mass flowmeter gives an accurate measurement of the massflow rate of a fluid. The flowmeter consists of a U-shaped oromega-shaped or curved tube in which the fluid circulates. The U-shapeis subjected to a lateral oscillation and the measurement of the phaseshift of the vibrations between the two arms of the U-shape gives arepresentation of the mass flow rate. However, its cost is relativelyhigh and when it is used at very low temperatures (liquid nitrogen at−196° C. for example) and with a fluid that has a density that variesenormously and that comprises a large portion in the gas phase, there isa need to highly insulate the system (efficient insulation such asvacuum insulation for example), and despite all that, the measurementsare distorted when the gas content exceeds a few percent by mass. Itwill also be noted that the measurement is often rendered impossiblewhen the velocity of the fluid is low or zero (in the first half of themeasurement range).

As may be observed, the measurement of the flow rate of a two-pleaseliquid and in particular the measurement of the flow rate of a cryogenicfluid with an acceptable accuracy is not easy to carry out with theapparatus currently available on the market.

The literature has then proposed other types of solutions, includingsystems based on the principle of measuring the level of a liquidflowing in a channel just before a restriction of the flow area. Thissystem, described in document U.S. Pat. No. 5,679,905 operates insubstance as follows: the two-phase fluid is firstly separated into agas phase which is not measured and a liquid phase, the flow rate ofwhich is measured. This liquid passes into a channel which has areduction in cross section at its outlet. The higher the flow rate, thehigher the level of liquid in the channel and a measurement of the levelin this channel makes it possible to deduce the instantaneous flow rate.As is observed, this system does not take into account the gaseous flowrate which, in certain applications, is negligible. On the other hand,this system makes it possible to measure, with a relatively goodaccuracy, the liquid flow rate without being disturbed by the gascontent, which is the desired objective.

It will be noted in passing that in order for this system to operatecorrectly, it must be well insulated from heat gains that could vaporizea portion of the insulated liquid and thus disturb the measurement ofthe level. This is why vacuum insulation is used in this system.

It will also be noted that in order for the system to operate, theremust be the presence of two phases in the flowmeter, which prohibits theoperation thereof with a subcooled liquid (pure liquid with no gasphase).

In the case where the measurement of the flow rates of liquid and of gasis necessary, use is sometimes made of a system that takes up the sameprinciple of separation of the phases before the flow rate measurement.

Thus, systems have the following device:

-   -   the two-phase liquid passes firstly into a phase separator that        separates the liquid phase from the gas phase;    -   the gas phase is sent to a volumetric flowmeter (of turbine type        for example) with a temperature compensation;    -   the liquid phase is also sent to a volumetric flowmeter (of        turbine type for example);    -   these two flow rate measurements are then converted into a mass        measurement and are added.

A priori, this device is more expensive than the preceding one, it maybe believed that it will be very accurate. In practice, it is observedthat the measurement of the liquid flow rate is marred by errors thatfluctuate depending on the pressure and temperature conditions of theliquid entering the flowmeter. These measurement errors are due to thepresence of gas in the liquid phase that passes through the flowmeter.Indeed, when the liquid leaves the phase separator in order to go to theflowmeter, a portion of liquid vaporizes, either because of heat gainsor because of the drop in pressure due to a rise of the liquid, orbecause of a drop in pressure due to the head loss created by theflowmeter itself.

Finally, in order to measure the flow rate of a cryogenic liquid, it isalso possible to avoid the problems cited above by creating pressure andtemperature conditions different from the equilibrium pressure (boilingrange). In this field, the method most commonly used is increasing thepressure of the liquid. In practice, a flowmeter will for example beinstalled at the outlet of a cryogenic pump (high-pressure side). Inthis case, the liquid is for example pumped into a tank where it is atequilibrium and it is raised in pressure by the pump, with almost noincrease in temperature. The pipes and the flowmeter that follow maythen create a head loss, this will not result in the liquid vaporizingprovided that the head loss is significantly lower than the increase inpressure created by the pump.

In this case, it is possible to use a conventional flowmeter of vortex,turbine or other type insofar as it withstands the low temperatures.

This technique is for example perfectly suitable for the flow ratemeasurement of nitrogen delivery trucks. It is reliable and has anacceptable cost insofar as the cryogenic pump is required for otherreasons.

On the other hand, when it is necessary to measure the flow rate ofliquid nitrogen at a point where there is no cryogenic pump, then thistechnique is no longer advantageous.

SUMMARY OF THE INVENTION

The present invention then endeavors to propose a novel, simple andreliable solution for measuring the flow rate of two-phase liquid/gascryogenic fluids that makes it possible to solve all or some of thetechnical problems mentioned above.

As will be seen in further detail in what follows, the solution proposedhere may be summarized thus:

-   -   The fluid arrives at a pressure that is variable, but generally        low (between 1 and 6 bar).    -   The fluid arrives under known or unknown pressure and        temperature conditions. In particular, the liquid phase may be        at equilibrium (boiling range).    -   The fluid may be composed of a liquid phase and of a gas phase        (two-phase liquid) in a variable proportion.    -   No device enabling the pressure to be increased (pump) is        required or available in the equipment.    -   The solution may be applied to any fluid when the latter has a        boiling point below the ambient temperature of the place where        the flowmeter is installed.

The device proposed comprises the following components:

-   -   A tank acting as a phase separator, this tank is advantageously        equipped with a liquid-phase level sensor, a liquid temperature        sensor and a gas-phase pressure sensor. It is understood that        according to the invention the use of a tank or volume where the        liquid is at rest in order to enable the separation of the        phases is favored, but it is also possible to use a large pipe        that will act as this separator.    -   A line for supplying two-phase fluid connected to the upper        portion of the tank.    -   A line connecting the top of the tank (therefore in        communication with the gas phase present at the top of the tank)        to an inlet of a 3-way valve and that passes through a gas flow        rate sensor. This line is equipped with a temperature sensor for        the gas circulating in this gas line.    -   A line connecting the bottom of the tank (therefore in        communication with the liquid phase stored in the bottom of the        tank) to another inlet of the 3-way valve mentioned above and        that passes through a liquid flow rate sensor.    -   A two-phase fluid outlet line connected to the 3^(rd) port of        the 3-way valve: the third port, outlet port of the valve,        combining the whole of these two inputs (mixture) in order to        send it for example to a downstream station that uses such a        fluid (cryogenic tunnel, drum or other for example).    -   The assembly is preferably thermally insulated.

As was explained above, in order for the measurement of the liquid phaseto be accurate, the liquid circulating in the liquid flow rate sensormust contain no (or virtually no) gas. Each gas bubble that passes intothe sensor results in a large measurement error.

In order to carry out an accurate measurement, this system in accordancewith the invention performs the following actions:

-   -   separation of the two phases of the fluid: the fluid arrives in        the tank which is in fact a phase separator. The liquid        accumulates naturally in the bottom of the tank and the gas in        the top portion of the tank.    -   measurement of the gas mass flow rate: this measurement is        carried out in a conventional manner that is well known to those        skilled in the art, the gas phase of the fluid passes through        the flowmeter present in the gas line, which measures the volume        flow rate of the gas. This flowmeter may for example be of        vortex, ultrasonic, turbine or calibrated orifice type. The        temperature probe measures the temperature of the gas, the        pressure sensor measures its pressure. By considering these two        measurements, for a given gas, the computer of the system        calculates the density of the gas passing into the flowmeter. By        thus having the volumetric flow rate and the density of the gas,        the computer then calculates, in a known manner, the mass flow        rate of the gas. The measurement of the mass flow rate of the        gas may also be carried out directly by means of a thermal or        Coriolis flowmeter.    -   Measurement of the liquid mass flow rate: it was said above that        this measurement is more difficult. Considering furthermore that        the liquid represents in certain applications such as cryogenic        applications more than 95% of the mass flow rate, it is on the        accuracy of this measurement that the (overall) measurement        accuracy of the apparatus depends. In order to carry out this        measurement without creating gas bubbles in a liquid, it is        performed, owing to the invention, in the following manner: on        leaving the tank, the liquid is at its liquid/vapor equilibrium        point. Any drop in pressure, however minimal it may be, gives        rise to the appearance of gas bubbles that substantially disturb        the measurement. A slight overpressure of the liquid is thus        created by installing the liquid flowmeter (present in the        liquid outlet line) at a sufficient distance below the tank,        preferably between 0.5 and 6 meters below the level of the tank,        typically around 1 meter. In other words, the tank is placed in        an “elevated” position (height “h”) relative to the liquid flow        rate sensor.    -   With this arrangement that makes it possible to create a very        slight overpressure due to the pressure head of the liquid in        the (thermally insulated) down pipe, the liquid arrives at the        flowmeter very slightly subcooled. Between the outlet of the        tank and the flowmeter, the temperature of the liquid does not        change but its pressure increases. It is then possible to        measure the volumetric flow rate of the liquid without creating        gas bubbles provided that the flow rate sensor does not give        rise to a head loss greater than the overpressure created by the        difference in height between the tank and the flowmeter. The        liquid flowmeter used may for example be of vortex, ultrasonic        or else turbine type. The volumetric flow rate thus measured is        then corrected by the density of the liquid in order to obtain        the mass flow rate. This liquid density is calculated by the        computer of the system owing to the temperature of the liquid        measured by the temperature sensor with which the tank is        equipped as mentioned above.    -   mixing of the two phases and outlet of the fluid: The gaseous        portion and the liquid portion that pass respectively through a        gas flowmeter and a liquid flowmeter are then mixed at the        three-way valve before leaving the apparatus.

According to the invention, this three-way valve is controlled accordingto one of the methods that will be explained better below, but thoseskilled in the art understand, in view of the aforegoing, that itrepresents a sort of “mixer tap” which mixes the gaseous nitrogen andthe liquid nitrogen that arrive thereat in proportions that it ispossible to dictate (and thus to dictate what leaves this valve at itsthird port).

The 3-way valve is slaved to the level of liquid in the tank via theinformation given by the liquid level sensor with which the tank isequipped.

By way of illustration, when the level of liquid in the tank is below alow setpoint value, the 3-way valve is positioned in order to raise thelevel: it allows gas through and closes the passage of the liquid. Thus,the level of liquid will rise in the tank.

Still by way of illustration, when the level of liquid is between a lowsetpoint value and a high setpoint value in the tank, the 3-way valve ispositioned to allow the liquid and the gas liquid through in an amountmore or less equal to 50/50. According to one of the embodiments, it maybe envisaged that the valve lets the liquid and the gas through inproportions that are different and even variable according to the valueof the level of liquid.

Still by way of illustration, when the level of liquid is above a highsetpoint value in the tank, the three-way valve is positioned in orderto lower the level: it closes the passage of the gas and lets throughthe liquid, thus the level of liquid will drop in the tank.

Thus, owing to such a method of control, the level of liquid in the tankremains between a low setpoint value and a high setpoint value and theliquid flow rate sensor only allows liquid with no gas bubbles through.

An accurate measurement of the mass flow rate of the gas and of theliquid is thus obtained. The computer may then either display the totalmass flow rate or the mass flow rates of the gas and liquid phasesseparately. Other methods of display may be envisaged in order forexample to display the energy equivalent of the fluid flow rate or todisplay the mass and volume contents of gas in the fluid.

The present invention thus relates to a flowmeter for two-phaseliquid/gas cryogenic fluids, comprising:

-   -   a liquid/gas phase separator, preferably consisting of a tank,        into the upper portion of which the cryogenic fluid is        introduced;    -   a liquid flow rate sensor, located in a liquid line in fluid        communication with the bottom portion of the tank, the tank        being placed in an elevated position in space relative to the        liquid flow rate sensor;    -   a gas line, in fluid communication with the top portion of the        tank, provided with a flow rate sensor for the gas phase        circulating in this gas line;    -   a three-way valve capable of recovering at two of its ports on        the one hand the downstream end of said gas line and on the        other hand the downstream end of said liquid line;    -   a device for measuring the level of liquid in the tank,        preferably comprising two level sensors, a bottom level sensor        and a top level sensor.

The invention could furthermore adopt one or more of the followingfeatures:

-   -   Use is made of a liquid flowmeter having a head loss that is as        low as possible.

It will be recalled that the suppliers of flowmeters give thisinformation in the specifications of the flowmeters that they sell.

Indeed, this disposition proves very particularly advantageous forensuring the fact that the head loss created by the liquid flowmeter isless than the pressure head between the tank and the liquid flowmeter,and thus for ensuring that it is not necessary to set up too great adistance between the tank and the liquid flowmeter, too great a distance(for example several meters) which would render the apparatus difficultto install in industrial premises.

By way of illustration, the head loss of the liquid flow rate sensor isless than the pressure head of the liquid between the bottom portion ofthe tank and the liquid flow rate sensor, and preferably less than 2meters of liquid height.

-   -   Use is made of a liquid flowmeter selected from the bottom        portion of its measurement range recommended by the        manufacturer. Working at low flow rate, the flowmeter then        creates a very low head loss. By way of illustration, at 30% of        its maximum flow rate, the head loss of the flowmeters on the        market is conventionally close to 10% of its maximum head loss        (flow rate divided by 3, head loss divided by 10).

By way of illustration, since the liquid flow rate sensor is sold foruse within a recommended range of flow rates, a range delimited by arecommended low flow rate and a recommended high flow rate, the flowrate of liquid circulating in the liquid line is always located within alow narrow range located between said recommended low flow rate and 30to 70% of said recommended high flow rate.

In other words, the chosen flowmeter is “oversized” by using a flowmeterin its low measurement range: by way of example, a flowmeter recommendedfor use in the range 300-3000 l/h will be used over its lower range offrom 300 to 1500 l/h.

It could be considered that this disposition has the drawback ofreducing the measurement range of the sensor. By way of illustration, asensor that initially has a measurement range that varies from 1 to 10(300 to 3000 l/h) can only be used with this technique over a range forexample of from 1 to 5. This may appear to be a factor that limits theuse of this technique.

-   -   It is then proposed to resort to an advantageous method of        implementation of the invention, according to which use is made        of the liquid flowmeter selected in a discontinuous manner in        accordance with the following two operating phases:        -   during a Phase 1 (which may be described as a “storage”            phase): the liquid flowmeter measures a flow rate greater            than the flow rate leaving via the 3^(rd) port of the 3-way            valve (and therefore greater than the flow rate leaving the            apparatus) by the fact that a “storage” of liquid is carried            out. Indeed, a portion of the liquid flow extracted from the            tank and that passes through the liquid flowmeter leaves the            apparatus (via the 3^(rd) port) while another portion (for            example in a 50/50 ratio) of the liquid flow extracted from            the tank and that passes through the liquid flowmeter is            allowed by the controller and the three-way valve to rise up            in the gas line (line used normally to make the gas descend            to the 3-way valve). The liquid thus accumulates in this gas            line. When the levels in the tank and in the line approach            one another, the pressure difference decreases and the flow            rate slows down, which is detected at the liquid flowmeter            when the value measured becomes less than the minimum flow            rate of its specifications (for example 300 l/h in the case            mentioned above). In this case, the automatic operation            (controller) of the system positions the 3-way valve so as            to block the passage of the liquid, the flow rate then            gradually changes from 300 to 0 l/h.        -   during a Phase 2 (which may be described as a “withdrawal”            phase: blockage of the flow in the liquid flowmeter and            “withdrawal” of the liquid accumulated in the gas line):            When the 3-way valve blocks the passage of the liquid and            allows through the gas, the liquid stored in the gas line is            then discharged via the outlet of the apparatus (3^(rd)            port). When there is no longer any liquid, the apparatus            then delivers gas and the level of liquid in the tank rises.            When this level passes above a threshold (high setpoint            value), the automatic operation then opens the valve on the            gas side and liquid side (for example 50/50) and Phase 1 is            then returned to.

Thus, by this alternating operation, the apparatus may measure flowrates in a very broad range, practically from 150 to 1500 l/h in ourexample, i.e. a range from 1 to 10 as desired at the start.

In other words, this discontinuous operation of the liquid flowmeterwith “storage” then “withdrawal” of the liquid makes it possible to onlyoperate the flowmeter in its nominal range while ultimately obtaining amean flow rate below the low value of the nominal range.

-   -   a data acquisition and processing system is available, which is        capable:        -   i) of acquiring a measurement of the level of liquid in said            separator;        -   j) of comparing this measurement to at least one setpoint            value of the level of liquid in the separator,        -   k) and, depending on the result of this comparison, of            performing feedback on the operation of the three-way valve            in order to dictate a ratio of the two fluids arriving            respectively in the liquid and gas branches of the valve,            and thus dictating the composition of the mixture leaving            its third port (feedback typically through a control system            that controls the three-way valve).    -   One or more pressure and temperature sensors are available,        which are capable of measuring the pressure in particular in the        gas phase of the tank and of measuring the temperature in the        liquid phase of the tank and where appropriate in the gas phase        of the tank and/or in the gas phase leaving through said gas        line.

The tests carried out with this system in accordance with the invention,used for the measurement of a liquid nitrogen flow rate, have shown thatit was possible to obtain an accuracy of the order of 3% over a flowrate range that varies from 1 to 10 which is very satisfactory fornumerous uses.

This system enables an accurate measurement of the flow rate of atwo-phase fluid without a pressurizing device, irrespective of thepressure and temperature conditions thereof at the feed point of thesystem. This system may be applied to any two-phase gas/vapor fluidprovided that, at the operating pressure of the system, the vaporizationtemperature of the fluid is below the ambient temperature of the placewhere the flowmeter is installed.

The invention will be better understood with the aid of the appendedfigures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic of the invention.

FIG. 2i is an illustration of operation of the 3-way valve where thesystem orders closure of the liquid port and opening of the gas port.

FIG. 2j is an illustration of operation of the 3-way valve where thesystem orders opening of the liquid port and gas port.

FIG. 2k is an illustration of operation of the 3-way valve where thesystem orders closure of the gas port and opening of the liquid port.

FIG. 3 is an illustration of operation of the invention where the levelof liquid is below a low level setpoint value.

FIG. 4 is an illustration of operation of the invention where the levelof liquid is between a low level setpoint value and a high levelsetpoint value.

FIG. 5 is an illustration of operation of the invention where the levelof liquid is above a high setpoint value.

FIG. 6 is an illustration of operation of the invention during a“storage phase” of the liquid.

FIG. 7 is an illustration of operation of the invention where the valveis actuated to block the passage of the liquid.

DETAILED DESCRIPTION OF THE INVENTION

The presence of the following components is seen in FIG. 1:

-   -   a liquid/gas phase separator 1, here consisting of a tank, into        the upper portion of which the two-phase cryogenic fluid is        introduced;    -   a liquid flow rate sensor 3, located in a liquid line 2 in fluid        communication with the bottom portion of the tank, the tank        being placed in an elevated position in space relative to the        liquid flow rate sensor;    -   a gas line 4, in fluid communication with the top portion of the        tank, provided with a flow rate sensor 5 for the gas phase        circulating in this gas line;    -   a three-way valve 6 capable of recovering at two of its ports        (A, B) on the one hand the downstream end of said gas line and        on the other hand the downstream end of said liquid line;    -   a measurement device 7 for measuring the level of liquid in the        tank, preferably comprising two level sensors, a bottom level        sensor and a top level sensor;    -   the tank is furthermore provided here with a pressure sensor 8        in the gas phase located in the top position of the tank but a        detailed description will not be given here of the various        pressure and temperature sensors that may be present in the        equipment, and that are capable of measuring the pressure in        particular in the gas phase of the tank and of measuring the        temperature in the liquid phase of the tank and where        appropriate in the gas phase of the tank and/or in the gas phase        leaving through said gas line, for reasons well known to those        skilled in the art.

FIGS. 2i, 2j, and 2k then it makes possible to better visualize theoperation of the three-way valve.

The inlet “G” denotes the inlet of the gas port, the inlet “L” denotesthe inlet of the liquid port and the port “S” denotes the outlet of thevalve.

Thus in FIG. 2i the case where the system orders the closure of theliquid port and the opening of the gas port has been illustrated.

In FIG. 2j the case where the system orders the opening of the liquidport and the opening of the gas port, for example in proportions of50-50, has been illustrated.

And FIG. 2k illustrates the case where the system orders the closure ofthe gas port and the opening of the liquid port.

The following figures that illustrate various operating phases of theflowmeter and especially control scenarios will now be examined.

As has been stated, the valve 6 is slaved to the level of liquid in thetank via the information given by the level sensor 7.

As FIG. 3 illustrates, when the level of liquid is below a low levelsetpoint value, the valve 6 is positioned by the controller in order toraise the level in the tank: it allows gas through and closes thepassage of the liquid. Thus, the level of liquid will rise in the tank.

When the level of liquid is between a low level setpoint value and ahigh level setpoint value (FIG. 4), the valve 6 is positioned to allowthe liquid and the gas through in amounts that are for examplesubstantially equal. In certain cases, it may be envisaged that thevalve lets the liquid and the gas through in proportions that aredifferent and even variable according to the value of the level ofliquid.

When the level of liquid is above a high setpoint value (FIG. 5), thevalve 6 is positioned in order to lower the level: it closes the passageof the gas and lets through the liquid which will lower the level ofliquid in the tank 1.

FIG. 6 illustrates the operation of the apparatus in Phase 1 explainedabove in the present description: this phase 1 is also referred to as“storage phase” of the liquid.

The sensor 3 measures a flow rate greater than the flow rate that leavesthe apparatus. Indeed, a portion of this flow rises up in the line 4(provided for making the gas descend to the valve 6) and accumulatestherein, until the levels in the tank and in the line 4 approach oneanother, the difference in pressure then decreases and the flow rateslows down.

The automatic operation of the system (controller) then detects thissituation (value measured by the valve 6 below a minimum flow rate) andthen positions the valve so as to block the passage of the liquid (FIG.7), in order “to withdraw” the liquid stored in the line 4, the valvethen delivers via its 3^(rd) outlet port liquid, then gas (when all theliquid is withdrawn), the level of liquid in the tank rises, until thislevel passes back between the high and low setpoint values, thecontroller will then reopen the valve on two sides (for example 50% gasside and 50% liquid side) etc., and phase 1 is returned to.

While the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. Accordingly, it is intendedto embrace all such alternatives, modifications, and variations as fallwithin the spirit and broad scope of the appended claims. The presentinvention may suitably comprise, consist or consist essentially of theelements disclosed and may be practiced in the absence of an element notdisclosed. Furthermore, if there is language referring to order, such asfirst and second, it should be understood in an exemplary sense and notin a limiting sense. For example, it can be recognized by those skilledin the art that certain steps can be combined into a single step.

The singular forms “a” “an” and “the” include plural referents, unlessthe context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means thesubsequently identified claim elements are a nonexclusive listing i.e.anything else may be additionally included and remain within the scopeof “comprising.” “Comprising” is defined herein as necessarilyencompassing the more limited transitional terms “consisting essentiallyof” and “consisting of”; “comprising” may therefore be replaced by“consisting essentially of” or “consisting of” and remain within theexpressly defined scope of “comprising”.

“Providing” in a claim is defined to mean furnishing, supplying, makingavailable, or preparing something. The step may be performed by anyactor in the absence of express language in the claim to the contrary.

Optional or optionally means that the subsequently described event orcircumstances may or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such a range isexpressed, it is to be understood that another embodiment is from theone particular value and/or to the other particular value, along withall combinations within said range.

All references identified herein are each hereby incorporated byreference into this application in their entireties, as well as for thespecific information for which each is cited.

What is claimed is:
 1. A flowmeter for two-phase liquid/gas cryogenicfluids adapted and configured to operate in a storage phase and operatein a withdrawal phase, said flowmeter comprising: a liquid/gas phaseseparator, an upper portion of which receives the cryogenic fluid; aliquid line in fluid communication with a bottom portion of theseparator; a liquid flow rate sensor located in the liquid line formeasuring a flow rate of the liquid phase cryogenic fluid circulatingtherein, the separator being placed in an elevated position in spacerelative to the liquid flow rate sensor; a gas line in fluidcommunication with the upper portion of the tank provided with a flowrate sensor for measuring a flow rate of the gas phase cryogenic fluidcirculating therein; a three-way valve having first, second, and thirdports, the first port being in fluid communication with a downstream endof said gas line, the second port being in fluid communication with adownstream end of said liquid line, said three-way valve beingconfigured and adapted to allow or disallow a flow of the gas phasecryogenic fluid via said first port and/or allow or disallow a flow ofthe liquid phase cryogenic fluid via said second port, the flow of thegas phase cryogenic fluid and/or liquid phase cryogenic fluid beingdirected by the third port to a downstream station that uses two-phaseliquid/gas cryogenic fluid; a bottom level sensor and a top level sensoradapted and configured to measure a level of liquid in the tank; acontroller adapted and configured: compare a level of liquid phasecryogenic fluid in the separator measured by the bottom and top levelsensor to at least one setpoint value for a level of liquid phasecryogenic fluid in the separator; depending upon a result of saidcomparison, control operation of said three-way valve to allow ordisallow the flow of gas phase cryogenic fluid into the first portand/or allow or disallow the flow of liquid phase cryogenic fluid intothe second port, thus dictating a composition of the two-phaseliquid/gas cryogenic fluid leaving the third port for the downstreamstation; determine a mass flow rate of the two-phase liquid/gascryogenic fluid by summing the measured flow rates of the gas and liquidphase cryogenic fluids flowing in the gas and liquid lines,respectively; during a storage phase in which the flow rate of theliquid phase cryogenic fluid measured by the liquid flow rate sensorexceeds a flow rate of cryogenic fluid being directed by the third portto a downstream station, operate said three-way valve to allow a portionof the liquid phase cryogenic fluid flow in the liquid line to rise intothe gas line and to allow a remaining portion of the liquid phasecryogenic fluid flow in the liquid line to exit the three-way valvethrough the third port; and during a withdrawal phase in which the flowrate of the liquid phase cryogenic fluid measured by the liquid flowrate sensor falls to a predetermined minimum flow rate specified by theliquid flow rate sensor, operate the three-way valve to disallow a flowof the liquid phase cryogenic fluid therethrough and to allow any of theliquid phase cryogenic fluid, that was previously allowed to rise intothe gas line, to drop back down to the three-way valve to exit throughthe third port.
 2. The flowmeter of claim 1, wherein the liquid flowrate sensor has a head loss that is less than a pressure head of theliquid between the bottom portion of the tank and the liquid flow ratesensor.
 3. The flowmeter of claim 2, wherein the liquid flow rate sensorhas a head loss equivalent to less than 2 meters of liquid height of theliquid phase cryogenic fluid.
 4. The flowmeter of claim 1, wherein theliquid/gas phase separator is a tank.